The production and use of cellulose food casings for the manufacture of frankfurters and the like is well known in the art. Automatic stuffing machines are available which stuff an uncooked meat or poultry emulsion into a long tubular casing while simultaneously forming the stuffed casing into links. The result is a long string of sausage links up to 40 to 50 meters long or longer.
At present, the only commercial process for manufacturing a cellulose casing for frankfurters involves the well-known viscose process. In the viscose process, a natural cellulose is contacted with a strong base to produce alkali cellulose. The alkali cellulose then is reacted with other chemicals to produce cellulose xanthate, a soluble cellulose derivative. The xanthate is dissolved in an aqueous solution of sodium hydroxide and the solution is extruded as a tube upwardly into an acid bath. The acid reacts with the xanthate to regenerate the cellulose in the form of a hydrated cellulose gel. Thus, with the viscose process, there is a first chemical reaction to create a soluble cellulose derivative and a second chemical reaction to regenerate the cellulose from the derivative. The gel then is washed, plasticized with a polyol such as glycerin and then is dried from a moisture content of 200% or more to a moisture content of less than 15% and preferably to about 5% to 10% based on the weight of dry cellulose in the casing or "bone dry gauge" (BDG).
Drying sets the properties of the tubular cellulose casing. Typically the dry casing then is passed through a steam chamber to remoisturized the cellulose to a moisture level of about 10 to 25% BDG. At this level, the casing is sufficiently pliable to permit further handling without damage.
More recently, a solution process has been adapted to the production of cellulose casing. Reference is made to U.S. Pat. Nos. 5,277,857 and 5,451,364 for details of the process. In brief, in the solution process the natural cellulose undergoes a direct dissolution by a cellulose solvent such as N-methyl-morpholine-N-oxide (NMMO). The resulting cellulose solution is thermoplastic in that it is solid at room temperature. The solution is extruded at about 100.degree. C. as a tube downwardly into a regenerating bath containing a non solvent for the cellulose such as water. In the bath the solvent is extracted from the extruded tube to precipitate or regenerate the cellulose as a hydrated cellulose gel. Thus, in the solution process there is no chemical reaction and the cellulose is non derivitized. For purposes of the present invention, "non derivitized" cellulose means a cellulose which has not been subjected to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces and/or hydrogen bonding. As in the viscose process, the tube of cellulose gel is washed (to remove residual solvent), dried to form a cellulose film and set properties and then the dried film is remoisturized.
The course of extrusion is downwardly through an air gap defined as the distance from the outlet of the extrusion die to the surface of the liquid in the regenerating bath. As disclosed in U.S. Pat. No. 5,277,857, there is a mandrel which depends from the extrusion die and extends to just below the level of liquid in the regenerating bath. This mandrel, which is disposed inside the extruded tube, includes a narrow stem and a lower portion that is larger in diameter than the stem. The enlarged lower end of the mandrel is a sizing portion as it functions to size the extruded tube by diametrically expanding it.
The extruded tube is highly viscous (3,000,000 to 11,000,000 centipoises) and will stick to the mandrel upon contact. To prevent this, a lubricating liquid, usually a dilute solution of the solvent, is introduced into the extruded tube through ports in the mandrel. Such an arrangement is disclosed in U.S. Pat. Nos. 5,759,478 and 5,766,540. As shown in these patents, the liquid pools around the stem of the mandrel just above the enlarged lower portion and forms an internal bath. The purpose of the internal bath is to start the extraction of solvent at the inner surface of the extruded tube and to facilitate the passage of the extruded tube over the enlarged lower portion of the mandrel. Lubrication is provided as liquid from the internal bath is drawn down and over the surface of the enlarged portion of the mandrel. The liquid drawn from the internal bath is continuously replaced at a rate which maintains the volume of the internal bath above the mandrel sizing portion relatively constant.
U.S. Pat. No. 5,451,364 discloses that casing properties are improved by increasing the length of the air gap to 12 inches (30 cm). U.S. Pat. No. 5,766,540 discloses use of even longer air gaps and suggests that passing the extrusion through an air gap of 50 cm or more may further improve properties.
However, an air gap length over 30 cm and up to 50 cm or more presents several problems both on start up of the extrusion operation and during continuous extrusion. The problems on start up are addressed by U.S. Pat. No. 5,766,540.
The extruded tube necks down as it is drawn through the air gap. Necking of the extruded tube is accommodated to some extent by having the mandrel stem a smaller diameter than the lower portion which sizes the tubing. Thus, as the extruded tube necks down it has clearance to avoid contacting the stem of the mandrel. However, it has been found that the diameter of the extruded tube in the region of the air gap decreases over time to the point where the tube can make contact with the stem of the mandrel. Such contact is highly undesirable because upon such contact the extruded tube will adhere to the mandrel stem and immediately interrupt the continuous extrusion operation.
There may be several reasons for the decrease in the diameter of the extruded tube over time. For example, one possible reason is that the volume of air within the extruded tube decreases over time due to leaks in the system that allows gases to pass up through the die. Another possible cause is that gases within the volume of the extruded tube are drawn downward by friction so as to pass from the volume above the enlarged portion of the mandrel to the volume below the enlarged portion. This is described in more detail in U.S. Pat. No. 5,759,478. The loss of gas volume also may be due to gradual dissolving of gasses in the liquid of the internal bath. For whatever reason, the excessive necking of the extruded tube is undesirable particularly where a long air gap is used for the reason noted above.
It has been found that the pressure within the extruded tube remains relatively constant as the tube diameter changes. Thus, the internal pressure is not indicative of the tube diameter. Accordingly, it is not practical to adjust the tube diameter by monitoring the air pressure within the extruded tube and then introducing air to expand the tube when the pressure falls below a predetermined level. Also, the extruded tube, as it exits the die, is molten. Accordingly, attempting to control the diameter by introducing air under pressure into the extruded tube will cause the tube diameter to expand. While such diametrical expansion is desirable in order to reestablish a clearance from the mandrel, the expansion increases the tube volume which again lowers the internal pressure prompting a further introduction air. The process would repeat until the extruded tube is excessively ballooned or blows out.
However, a narrowing or collapse of the extruded tube decreases the volume within the extruded tube and the level of the internal bath above the mandrel sizing portion rises. Thus, the level of the internal bath is indicative of the diameter of the extruded tube. In a preferred method of operation, the level of the internal bath is above the level of the regenerating bath. Raising the level of the internal bath, as would occur as the extruded tube narrows or necks down, causes other problems (not discussed herein) and is to be avoided.
As noted above, a rise in the level of the internal bath is indicative of a narrowing of the diameter of the extruded tube in the area of the air gap. Accordingly, the liquid level can be used to trigger a response to increase the tube diameter. However, due to the small clearance already existing between the mandrel shaft and the extruded tube surrounding the shaft, the insertion of level indicators into the tube is not practical. Also, both the extruded tube and internal bath are relatively clear so it is difficult for an optical sensor to distinguish the level of the internal bath based on a change in color or opacity from the clear tubing as the level of the internal bath rises.
Accordingly, while it is desirable to automate the control of the diameter of the extruded tube, it is due to the problems noted above that heretofore the adjustment of the diameter and bath level has been manual. In this respect, the skill of the operator was relied upon to track the level of liquid in the internal bath. When the diameter of the extruded tube decreased below and acceptable limit, the operator triggered a solenoid for introducing one or more pulses of air into the extruded tube. This expanded the extruded tube which in turn dropped the level of the internal bath. On the rarer occasion when the diameter of the extended tube increased above an acceptable limit, the operator would bleed air from the extended tube to decrease its diameter.
Accordingly, one object of the present invention is to provide an automated method and apparatus for maintaining the extruded tube in the area between the extrusion die and the enlarged lower end of the mandrel at a desired diameter.
Another object is to provide a method and apparatus that is not pressure responsive for adjusting the air volume within the extruded tube in order to prevent excessive necking of the tube as it is drawn through an air gap.
A further object is to provide a method and apparatus for adjusting the diameter of the extruded tube in response to a change in the level of a pool of liquid within the extruded tube.